The invention relates to a high-pressure sodium vapor discharge lamp provided with a sealed ceramic discharge vessel which has over a length L a substantially constant inner diameter,
in which discharge vessel electrodes are arranged opposite to each other at a relative distance D and are connected to a respective current-supply conductor, which extends through the wall of the discharge vessel to the exterior,
which discharge vessel has a filling which comprises sodium and rare gas,
which lamp consumes during operation a power of at most 50 W and emits light having a colour temperature of at least 2250 K. Such a lamp is known from British Patent Specification No. 20,83,281 and corresponding U.S. Pat. No. 4,475,061.
A lamp of this kind can be used to replace an incandescent lamp. The lamp emits "white light". In general, it holds for the color temperature (T.sub.c) that 2250.ltoreq.T.sub.c .ltoreq.2750 K. The range in the color triangle (C.I.E. chromaticity diagram) within which the light of a high-pressure sodium discharge lamp is designated as "white" is bounded by straight lines through points with the coordinates (x, y): (0.468; 0.430), (0.510; 0.430), (0.485; 0.390) and (0.445; 0.390). According to more stringent standards based on a better acceptance of the light by testees, the light is designated as "white" when its color point lies in a range of the color triangle bounded by the lines x=0.468, x=0.490, y=0.408 and y=0.425. The color temperature then lies between about 2300 and about 2700 K and the general color rendering index (Ra.sub.8) lies between about 70 and about 85.
Lamps of this kind are attractive as substitutes for incandescent lamps because of their a few times longer life, their a few times higher efficiency, their luminius flux corresponding to that of the larger incandescent lamps (about 60-200 W) and because of the fact that their light can be readily concentrated.
A disadvantage of lamps of this kind is that their efficiency is lower than that of high-pressure sodium lamps emitting yellow light (T.sub.c .apprxeq.1800-2000 K), i.e. lower as the color temperature is higher. Furthermore, the efficiency decreases with decreasing power.